1. Field of the Invention
This invention relates to stripping gold from activated carbon at ambient temperature by contacting the gold-loaded activated carbon with a strong base and then an organic solvent and subsequently recovering the stripped gold from the solvent using ion exchange technology.
2. Description of Prior Art
In the mining industry, gold ore is most commonly leached with a cyanide solution and the gold is recovered from the cyanide lixiviant by adsorption on activated carbon. The most common commercial techniques for elution of gold cyanide from activated carbon are the Zadra (Zadra et al., U.S. Pat. No. 2,588,450, issued Mar. 11, 1950) and Anglo procedures. (Davidson and Duncanson, "Desorption of Gold from Activated Carbon with Deionized Water," J. South Afr. Instr. Min. Metall., 77(12), pp 254-261, (1977); Davidson and Veronese, "Further Studies on the Elution of Gold from Activated Carbon using Water as the Eluant," J. South Afr. Instr. Min. Metall., 79(1), pp 437-495, (1979).
In the Zadra procedure, hot solutions of 1% weight/volume (w/v) sodium hydroxide and 0.2% w/v sodium cyanide are recycled through a gold cyanide-loaded activated carbon bed for up to 72 hours at 95.degree.-100.degree. C. to desorb Au(CN).sub.2.sup.-. More recently, a modified Zadra procedure operating at 140.degree. C. in a pressurized system has reduced elution time to 10-12 hours.
In the Anglo elution procedure, gold cyanide-loaded activated carbon is contacted with 5% w/v sodium cyanide and 1% w/v sodium hydroxide followed by elution for 8-12 hours with hot deionized water at 100.degree.-120.degree. C. While both the Zadra and Anglo procedures are effective in stripping gold from activated carbon, those procedures suffer from high energy consumption, high capital costs for pressurized operations, long elution times and the use of high concentrations of sodium cyanide.
Other attempts to strip gold bound to activated carbon have been directed toward development of methods performed at lower temperatures that desorb gold faster than either the Zadra or Anglo elution procedures.
D. M. Muir, W. Hinchliffe, N. Tsuchida and Ruane, M., "Solvent Elution of Gold from C.I.P. Carbon, Hydrometallurgy 14, 47-65 (1985) (herein after Muir et al. (1985a)) reported that, in the presence of 10 g/L of sodium cyanide, either 40 percent volume/volume (v/v) aqueous acetone or 40 percent (v/v) aqueous acetonitrile desorbs gold in approximately eight hours at temperatures of 25.degree. to 70.degree. C. However, temperatures between 50.degree. and 70.degree. C. offered much faster and more efficient gold desorption. Muir et al. state that organic solvents by themselves or mixed with water do not desorb gold from activated carbon unless either sodium cyanide is added to the solvent or the gold-loaded activated carbon is soaked with sodium cyanide prior to contact with the solvent.
Muir et al. (1985a) further reported that the activity of the activated carbon for gold loading decreased upon successive gold-loading/stripping cycles (with 40 percent aqueous acetonitrile containing 10 g/L of NaCN) to the point that, after 5 cycles, the relative activity of the activated carbon dropped to about 50 percent of its original value. This is similar to the loss of activated carbon activity observed upon successive loading/stripping cycles using either the Anglo or Zadra elution procedures. Muir et al. (1985a) noted that some of the gold-binding activity of the activated carbon could be restored by a steam treatment, but the activity was not restored to levels of fresh activated carbon. It was also noted that stripping of gold-loaded activated carbon with aqueous acetone containing NaCN produced irreversible loss in activated carbon activity for further gold loading.
Patents have been issued to Parker et al. (A. V. Parker and D. M. Muir, "Composition for Stripping Gold or Silver from Particulate Materials," U.S. Pat. No. 4,427,571 issued Jan. 24, 1984) and to Heinen et al. (H. J. Heinen, D. G. Peterson and R. E. Lindstrom, "Desorption of Gold from Activated Carbon," U.S. Pat. No. 4,208,378 issued Jun. 17, 1980) for stripping gold from activated carbon using either aqueous solutions of nitriles containing sodium cyanide or sodium thiocyanate or aqueous solutions of alcohols containing sodium cyanide or sodium thiocyanate.
D. M. Muir, W. D. Hinchliffe and A. Griffin, "Elution of Gold from Carbon by the Micron Solvent Distillation Procedure," Hydrometallurgy. 14, 151-169, (1985) (herein after Muir et al. (1985b)) proposed a gold desorption procedure by pretreatment of gold-laden activated carbon with a solution of sodium cyanide and sodium hydroxide followed by elution with one of methanol, ethanol or acetonitrile vapors and condensate at 65.degree.-80.degree. C. Using this procedure, the gold cyanide was stripped in 4-6 hours. However, activated carbon activity was lost through subsequent gold-loading/stripping cycles.
F. Espiell, A. Roca, M. Cruells and C. Nuneg, "Gold Desorption from Activated Carbon with dilute NaOH/Organic Solvent Mixture," Hydrometallurgy 19 321-333 (1988) (herein after Espiell et al. (1988)) examined gold desorption from activated carbon using mixtures of NaOH (20 g/L) and 50% aqueous organic solvents at 30.degree. C. The acetone-water-hydroxide system was reportedly most efficient at gold desorption with over 90 percent of the gold being stripped in less than 40 minutes. However, a loss in gold-binding activity was observed over several loading/stripping cycles. This loss in activity was attributed to the inability of the acetone solvent system to strip the gold most strongly adsorbed to the activated carbon.
D. D. Fisher, "Process for Eluting Adsorbed Gold and/or Silver Values from Activated Carbon," U.S. Pat. No. 3,935,006 issued Jan. 27, 1976 asserted that at ambient temperatures, aqueous solutions of water-soluble alcohols containing a strong base such as sodium hydroxide were capable of stripping over 98 percent of the gold adsorbed on activated carbon. In contrast, Espiell et al. (1988) determined that a mixture of a strong base in aqueous methanol is one of the poorest gold eluants for gold-loaded activated carbon. Similarly, Muir et al. (1985b) determined that, at room temperature, aqueous methanol in the presence of sodium cyanide was one of the poorest eluants of gold from activated carbon.
W. T. Yen and R. H. Pinred, "Carbon Stripping at Ambient Temperatures," in Precious Metals 1989, Proceeding of the Thirteenth International Precious Metals Institute Conference, International Precious Metals Institute, Allentown, Pa., pp. 261-274 (1989) pre-soaked gold-loaded activated carbon with a solution containing NaCN and NaOH, and then stripped the gold at room temperature by elution with a solution containing NaCN, NaOH and 40% (v/v) acetonitrile. They reported that 98 percent of the gold was stripped from activated carbon in six hours using this method. However, loss of activated carbon activity for gold loading amounted to 31 percent after contact with the caustic cyanide/acetonitrile solution.
Muir et al. (1985b) determined that a 40 percent aqueous solution of either acetone or acetonitrile, which solution contained NaCN and the gold stripped from the activated carbon, could be treated by electrowinning to recover metallic gold. However, the fire hazards of electrowinning caused by the flammable organic solvent and solvent losses due to evaporation required the use of an expensive sealed diaphragm electrowinning cell which contained a membrane to separate the anolyte and catholyte. Although metallic gold could be recovered by electrowinning from the 40 percent acetonitrile solution; temperature, current density and gold concentration were critical in effective, efficient, recovery of gold.
In summary, previous commercial methods devised for stripping dicyanoaurate(I) anions bound to activated carbon have involved: 1) a high temperature/pressure pre-soak of the gold-laden activated carbon with aqueous NaCN/NaOH solutions followed by stripping with hot deionized water; 2) a high temperature pressure strip with aqueous NaCN/NaOH; or 3) organic solvents which contain aqueous NaCN/NaOH. These methods all require the use of NaCN in the stripping process. The previous methods developed for stripping the gold cyanide anion from activated carbon suffer from high energy costs (i.e., stripping at high temperatures), from lengthy stripping times, from the use of high concentrations of environmentally objectionable sodium cyanide, from loss of activated carbon gold-binding activity over successive loading-stripping cycles, or from the fire hazards associated with electrowinning gold cyanide from solutions containing organic solvents. Thus a safe, rapid, ambient temperature process for stripping gold cyanide from activated carbon would be useful for subsequent recovery of metallic gold from aqueous solutions containing Au(CN).sub.2.sup.- :